The background art of the present invention is now explained taking an excavator as an example.
A general excavator includes, for example, as shown in FIG. 6, a crawler-type base carrier 1, an upper slewing body 2 mounted thereon so as to be capable of being slewed about an X axis that is perpendicular to the ground, and an excavating attachment 3 which is attached to the upper slewing body 2. The excavating attachment 3 includes a boom 4 capable of being raised and lowered, an arm 5 mounted at the tip of the boom 4, a bucket 6 mounted at the tip of the arm 5, and respective cylinders (hydraulic cylinders) for moving the boom 4, the arm 5 and the bucket 6, namely, a boom cylinder 7, an arm cylinder 8 and a bucket cylinder 9. As hydraulic actuators other than the cylinders 7 to 9, further provided are left and right travel motors for driving the left and right crawlers of the base carrier 1, and a slewing motor configured from a hydraulic motor for driving the upper slewing body 2 to slew it.
In order to operate the actuators, used are a hydraulic pump as a hydraulic power source thereof, a remote control valve as a operation device, and a control valve which is operated so as to control supply of hydraulic fluid from the hydraulic pump to the actuators and discharging the hydraulic fluid from the actuators, based on the operation applied to the operation device; these control the operating direction and the operating speed of the actuators. With respect to the respective actuators, build are respective actuator circuits, each of which is provided with a main relief valve (hereinafter simply referred to as the “relief valve”) between the pump discharge pipe-line and the tank to prevent the circuit piping and equipment from being damaged due to high pressure, the main relief valve being adapted to be opened when the circuit pressure exceeds the relief pressure to let the hydraulic fluid to the tank.
With regard to the slewing circuit for driving the slewing motor, of the actuator circuits, a variable displacement pump is used as the hydraulic pump, and so-called positive control of increasing the discharge flow rate of the pump (hereinafter referred to as the “pump flow rate”) with increase in the amount of the operation applied to the remote control valve (hereinafter referred to as the “slewing operation amount”) is adopted from time to time. In the foregoing case, the difference between the pump flow rate determined based on the slewing operation amount and the motor flow rate actually used in the rotation of the slewing motor, i.e., the slewing flow rate, corresponds to an excess flow rate which is the flow rate of the hydraulic fluid to be let from the relief valve to the tank, and the energy efficiency of the hydraulic pump is inferior as the excess flow rate, i.e., the relief flow rate, is greater.
In order to inhibit the excess flow rate, there is conventionally known an art of estimating the relief flow rate from the pump pressure during slewing and the pressure characteristics of the relief valve, and controlling the pump flow rate so as to make the estimated value be 0, as described in Patent Document 1. According to the art of thus “estimating” the relief flow rate, i.e., the excess flow rate, based on the pressure characteristics of the relief valve as described above, it is difficult to obtain a highly accurate estimated value, because the pressure characteristics itself, which are used as the grounds for estimation, will vary due to the variation in the respective relief valves or the variation caused by temperatures. This may lead to the deterioration in the work efficiency caused by the insufficient slew speed associated with the insufficient flow rate, or contrarily lead to the deterioration in the power conservation effect caused by the increase in the excess flow rate associated with the excessive flow rate.    Patent Document 1: Japanese Patent Application Publication No. 2004-225867